(1) A molecular replacement solution of the crystal structure of unliganded Fab of HyHEL-10 has been obtained and is being refined. (2) The molecular replacement analysis of the crystal structure of the Fab of CC49, a murine monoclonal antibody against solid adenocarcinoma, is being pursued. (3) Crystals of a single-chain construct containing the VL and VH of CC49 fused to human IgG1 CH2 and CH3 have been obtained. (4) Crystallographic studies on various other proteins have been initiated. (5) Models of the extracellular portions of the human and murine low- affinity IgE receptors (CD23) have been built. (6) The binding of various viral and self peptides to the murine class I MHC antigen, H-2Dd, has been modelled.